(a) Field of the Invention
The present invention relates to a microscope objective and, more particularly, to a microscope objective with a magnification about 40.times. of which the working distance is long and aberrations are favourably corrected up to the marginal portion of field.
(b) Description of the Prior Art
Known microscope objectives with a magnification about 40.times., long working distance and aberrations favourably corrected up to the marginal portion of the field are disclosed, for example, in Japanese published unexamined patent application No. 33646/72 etc. However, the working distance of those known microscope objectives is about 70% of the focal length, and this is insufficient when using the microscope objectives in such manner as to give some operation to the specimen by observing it through a microscope. Besides, even when using the microscope objectives in other manners, a working distance longer than the above is more preferable because it is then convenient for use.
On the other hand, microscope objectives have such tendency that aberrations abruptly become unfavourable when it is attempted to make the working distance long by keeping the same degree of NA and, consequently, it becomes difficult to obtain a favourable quality of image. Therefore, in known microscope objectives, a meniscus lens having a concave surface with strong curvature on the image side is arranged in the rear lens group in order to make the working distance long. However, even when the above-mentioned arrangement is made, aberrations abruptly become unfavourable when the working distance is made longer than a certain degree, and it becomes difficult to correct them.
Due to the above-mentioned reason, there is no known microscope objective of which the working distance is very long and, at the same time, aberrations are favourably corrected up to the marginal portion of field.